1. Field of the Invention
The invention relates to the field of audio peak limiters.
2. Prior Art
Many techniques are used for peak limiting of signals including audio frequency signals. Among them are fast-attack, slow-release automatic gain control (AGC) amplifiers, diode audio-frequency clippers, diode radio-frequency clippers, and fast-attack slow-release AGC amplifiers employing delay lines. Each technique has its own audible, subjective strengths and weaknesses.
Particularly when combined with delay line techniques, AGC techniques can be configured to have no audible harmonic or intermodulation distortion. However, they avoid such distortion by using relatively little non-linear modification of the audio waveform. Because such a system only senses peak level (which has little or no correlation with loudness), if the peak-to-RMS ratio of the signal varies significantly, then highly unnatural loudness variations (sometimes called "pumping" and "hole-punching") can result.
Audio frequency clippers are extremely simple: a pair of back-to-back zener diodes which simply clip off any peaks exceeding a given threshold is one example. This technique can severely modify the audio waveform in a non-linear way, producing audible harmonic and/or intermodulation (IM) distortion when used to excess. However, because this technique acts only on instantaneous peaks, it causes no significant loudness variation and is therefore frequently used. Generally clipping is preceded by automatic level control circuitry designed to control the amount of distortion produced by the clipping thereby limiting distortion to inaudible (or at least esthetically acceptable) levels. U.S. Pat. No. 4,208,548 discloses a system for controlling the amount of distortion produced by an audio frequency clipper.
The radio frequency (RF) clipper has proven popular for processing voice signals in shortwave and communications applications. Here, the audio frequency signal is modulated into a single-sideband suppressed-carrier RF signal. This RF signal is clipped and the clipped signal is then demodulated. An interesting property of this technique is that no harmonic distortion is produced with a pure tone, since the first harmonic produced by the clipping is located at an integral multiple of the RF carrier frequency. If the carrier frequency is 1 MHz, the harmonics occur at 2 MHz, 3 Mhz, etc. These harmonics are eliminated upon demodulation and ordinary filtering. (Upon demodulation, the 2 MHz harmonic becomes 1 MHz: still well outside the audio range.)
Unfortunately, with ordinary program material (consisting of many frequencies simultaneously), RF clipping can produce IM distortion which is even more severe than that produced by audio frequency clipping. The peak level of the output must be instantaneously constrained to a given level, thus causing the distortion. If no harmonic distortion is permitted, the waveform modification necessary to control the peak level must be entirely at the expense of added IM distortion. Subjectively, this technique sounds far better than audio frequency clipping on voice and substantially worse than audio frequency on music. This is because when voice is clipped, the objectionable audible distortion is primarily harmonic distortion in the frequency range above the fundamental frequencies of voice. These harmonics fall in the frequency range to which the ear is most sensitive (1-5 kHz), and in which there is little naturally-occurring energy in voice to mask such harmonics. Accordingly, the harmonic-distortion suppression properties of the RF clipper are very useful for voice.
In contrast, most music is much denser spectrally than is voice. Harmonic distortion can often add a pleasing brightness to music because the harmonics are harmoniously related to the music, and because naturally-occurring harmonics tend to mask the addition of moderate amounts of added harmonic distortion. But IM distortion is not harmonious, and therefore always degrades the subjective quality of music. Therefore, audio frequency clipping (particularly very "hard" clipping characterized by a highly linear transfer curve up to the clipping threshold) tends to sound better than RF clipping on most music. Exceptions occur with instruments having simple spectra with few high-frequency components to mask unnaturally-induced harmonics. Examples of such instruments are grand piano, harp, nylon-stringed acoustic guitar, and Fender-Rhodes electric piano.
The present invention behaves like an RF clipper below a certain frequency (4 kHz in the preferred embodiment), and like an audio frequency clipper above this frequency. Accordingly, no harmonic distortion is produced by input material below 4 kHz and the advantages of RF clipping are achieved on voice, "dull-sounding" instruments, and other such program material with little naturally-occurring high frequency energy to mask harmonic distortion. Conversely, in the case of most music (particularly in a system employing high frequency preemphasis, such that peak limiting must be effected after such preemphasis), most of the energy to be controlled by the peak limiter is usually located above 4 kHz. By processing this frequency band with the equivalent of audio frequency clipping, minimum difference-frequency IM distortion (which is particularly objectionable when listened to after high-frequency deemphasis in a receiver) is obtained.
The closest prior art known to Applicant is that shown in an article entitled "Decomposition of Nonlinear Operators into `Harmonic` Components, with Applications to Audio Signal Processing", Electronics Letters, Vol. 12, No. 7, pp. 23-24 (Jan. 8, 1976) by M. A. Gerzon, (hereinafter referred to as "the Gerzon article"). With respect to the crossover network of FIG. 6 of this application, the closest prior art known to Applicant is "A Family of Linear-Phase Crossover Networks of High Slope Derived by Time Delay", by Lipshitz and Vanderkooy, a paper presented at the 69th Convention of the Audio Engineering Society (May 12-15, 1981 at Los Angeles, Calif.).